Influence de la composition et de la temperature sur les reactions electrochimiques et le domaine de stabilite electrochimique des borates de sodium fondus

The oxidation and reduction reactions, limits of the electrochemical stability range, were determined by cyclic voltammetry in molten sodium borates 3 Na₂OB₂O₃ (I), Na₂O2B₂O₃ (II) and Na₂O4 B₂O₃ (III) between 750°C and 1000°C.These reactions are: on gold electrode the oxidation of O^2? ions to oxygen and the reduction of borate ions to elemental amorphous boron; on platinum electrode the oxidation and passivation of the metal and the reduction of borate ions to form platinum borides Pt₃B and Pt₂B.The electrochemical stability ranges are respectively in solvent I 0.67 V (750°C), 0.97 V (850°C), 0.92 V (1000°C); in solvent II 1.94 V (750°C), 1.92 V (850°C), 1.85 V (1000°C); in solvent III 1.85 V (850°C), 1.75 V (1000°C).     

Synthesis and Characterization of Silicon Carbide Powders Converted from Metakaolin‐Based Geopolymer

Silicon carbide (SiC) ceramic powders were synthesized by carbothermal reduction in specific geopolymers containing carbon nanopowders. Geopolymers containing carbon and having a composition M₂O·Al₂O₃·4.5SiO₂·12H₂O+18C, where M is an alkali metal cation (Na⁺, K⁺, and Cs⁺) were carbothermally reacted at 1400°C, 1500°C, and 1600°C, respectively, for 2 h under flowing argon. X‐ray diffraction and microstructural investigations by SEM/EDS and TEM were made. The geopolymers were gradually crystallized into SiC on heating above 1400°C and underwent significant weight loss. SiC was seen as the major phase resulting from Na‐based geopolymer heated to ≥1400°C, even though a minor amount of Al₂O₃ was also formed. However, phase pure SiC resulted with increasing temperature. While a slight increment of the Al₂O₃ amount was seen in potassium geopolymer, Al₂O₃ essentially replaced cesium geopolymer on heating to 1600°C. SEM revealed that SiC formation and a compositionally variable Al₂O₃ content depended on the alkaline composition. Sodium geopolymer produced high SiC conversion into fibrous and globular shapes ranging from ~5 μm to nanosize, as seen by X‐ray diffraction as well as SEM and TEM, respectively.     

Determination of Retained B2O3 Content in ZrB2‐30 vol% SiC Oxide Scales

The composition of the borosilicate glass layer formed during oxidation of ZrB₂‐30 vol% SiC was determined to elucidate the extent of B₂O₃ retention in the oxide during high‐temperature oxidation. Oxidation was conducted in stagnant air at 1300°C, 1400°C, and 1500°C for times between 100 and 221 min. Specimens were characterized using mass change and scanning electron microscopy. After oxidation, the borosilicate glass layer was dissolved from the specimens sequentially with deionized H₂O and HF acid, to analyze the glass composition using inductively coupled plasma optical emission spectrometry. It was found that the average B₂O₃ content in the glass scale ranged from 23 to 47 mol%. Retained B₂O₃ content in the bulk of the glass decreased with increasing temperature, confirming increased volatility with temperature. Boron depth profiles were also obtained in the near surface region using X‐ray photoelectron spectroscopy and energy dispersive spectroscopy. The measured B concentrations were used to estimate the B₂O₃ concentration profile and B diffusion coefficients in the borosilicate glass. Implications for the ZrB₂‐SiC oxidation process are discussed.     

Ultra‐low Sintering Temperature Microwave Dielectric Ceramics Based on Na2O‐MoO3 Binary System

The compounds in Na₂O‐MoO₃ system were prepared by the solid‐state reaction route. The phase composition, crystal structures, microstructures, and microwave dielectric properties of the compounds have been investigated. This series of compounds can be sintered well at ultra‐low temperatures of 505°C–660°C. The sintered samples exhibit good microwave dielectric properties, with the relative permittivities (ε_r) of 4.1–12.9, the Q × f values of 19900–62400 GHz, and the τ_f values of ?115 ppm/°C to ?57 ppm/°C. Among the eight compounds in this binary system, three kinds of single‐phase ceramics, namely Na₂MoO₄, Na₂Mo₂O₇ and Na₆Mo₁₁O₃₆ were formed. Furthermore, the relationship between the structure and the microwave dielectric properties in this system has been discussed. The average Na^I‐O and Mo^VI‐O bond valences have an influence on the sintering temperatures in Na₂O‐MoO₃ system. The large valence deviations of Na and Mo lead to a large temperature coefficient of resonant frequency. The X‐ray diffraction and backscattered electron image results show that Na₂MoO₄ doesn't react with Ag and Al at 660°C. Also, Na₂Mo₂O₇ has a chemical compatibility with Al at 575°C.     

Interaction Study of Yttria‐Based Glasses with High‐Temperature Electrolyte for SOFC

The chemical interaction study of AO–SiO₂–B₂O₃–Y₂O₃ (A = Ba, Sr) (BaY, SrY) glass with high‐temperature electrolyte yttria‐stabilized zirconia (YSZ 8 mol%) is reported as a function of different heat treatment durations. The as‐prepared glass with 10 mol% of yttria shows limited amount of crystallization at 800 °C. Due to this yttria‐based glasses BaY and SrY have been chosen to make diffusion couples with high‐temperature electrolyte and interconnect material. These diffusion couples have been heat treated at 850 °C, for 100, 200, and 500 h. The heat‐treated diffusion couples have been characterized using X‐ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Microstructural analysis of diffusion couples shows absence of any undesired oxides and detrimental reaction products at the interface. The glass has shown good bonding characteristics and absence of cracks, pores, or any kind of delamination from YSZ. Apart from this, SrY and BaY glass seals have also shown good adhesion characteristics with Crofer 22 APU, even after 500 h at 850 °C. The morphology and microstructure of the glass matrix suggest limited amount of devitrification in the glass.     

Na2SO4 deposit-induced hot corrosion of BN-coated α-SiC

BN coated α-SiC was exposed to ~2.5 mg/cm² of Na₂SO₄ for 0.75-144 hours between 700°C and 1100°C in a 0.1% SO₂-O₂ gaseous environment. A combination of SEM, EDS, ICP-OES, XRD, and optical profilometry was used to determine that BN induced a previously unobserved form of Na₂SO₄ deposit-induced hot corrosion of SiC below the melting temperature of Na₂SO₄ (T_m = 884°C). Above the melting temperature of Na₂SO₄ BN did little to alter the hot corrosion behavior of SiC, due to B₂O₃ volatility. Substrate attack below the melting temperature of Na₂SO₄ was found to result in rivulet recession features, whereas deep, localized pitting occurred above the melting temperature of Na₂SO₄. Initially, the mixture of Na₂SO₄ and BN led to the formation of a Na₂O- and B₂O₃-containing liquid silicate and accelerated corrosion which after prolonged exposure, resulted in the partial crystallization into a lath-like tridymite and cristobalite mixture. Prolonged exposure resulted in a flattening of SiC consumption rates after exposure for several hours at 1000°C and after several days at 800°C, resulting in decreased SiC oxidation/corrosion kinetics.     

RELATION OF ELECTRICAL CONDUCTIVITY TO CHEMICAL COMPOSITION OF GLASS*

The electrical conductivity of some glasses in the soda-lime-silica system was measured at 400 °C. with an apparatus which is described. The specific resistance at this temperature ranged from 0.06 to 5.0 × 10⁵ ohms per cc. Measurements were made on other glasses containing equivalent amounts of various oxides incorporated in a parent soda-lime-silica glass. It was found that an increase in resistance was produced by additions of MnO, ZnO, B₂O₃, Fe₂O₃, BaO, PbO, TiO₂, and K₂O, the resistances increasing in this order. A decreased resistance was produced by Na₂O, CaO, and Al₂O₃. Previously published data on the power factors of these glasses are reviewed.     

The solubility of magnesium metal and the recombination reaction in the industrial magnesium electrolysis

The solubility and dispersion of Mg in the systems MgCl₂, MgCl₂-NaCl and MgCl₂-KCl has been studied. An extensive study of Mg solubility in MgCl₂ did not show any influence on the content of MgO or added NaOH or Na₂B₄O₇. At 750°C the solubility was found to be 0.16 mol%. The solubility increased with increasing temperature and decreased on the addition of NaCl or KCl. In 25 mol% NaCl-75 mol% MgCl₂ the solubility was as low as 0.02 mol% at 815°C. The amount of Mg dispersed was found to be negligible with the present experimental set-up. The results are discussed with reference to the back reaction between Mg and Cl₂ in the industrial electrolysis. It seems likely that the rate determining step in this reaction is the rate of formation of small metal particles in the melt.     

The Effects of Na2O and SiO2 on Liquid Phase Sintering of Bayer Al2O3

To determine how grain‐boundary composition affects the liquid phase sintering of MgO‐free Bayer process aluminas, samples were singly or co‐doped with up to 1029 ppm Na₂O and 603 ppm SiO₂ and heated at 1525°C up to 8 h. Na₂O retards densification of samples from the onset of sintering and up to hold times of 30 min at 1525°C compared to the undoped samples, but similar to the as‐received, MgO‐free Al₂O₃, Na₂O‐doped samples sinter to 98% density with average grain sizes of ~3 μm after 8 h. Increasing SiO₂ concentration significantly retards densification at all hold times up to 8 h. The estimated viscosities (20?400 Pa·s) of the 0.3 to 1.8 nm thick siliceous grain‐boundary films in this study indicate that diffusion greatly depends on the composition of the liquid grain‐boundary phase. For low Na₂O/SiO₂ ratios, densification of Bayer Al₂O₃ at 1525°C is controlled by diffusion of Al³⁺ through the grain‐boundary liquid, whereas for high Na₂O/SiO₂ ratios, densification can be governed by either the interface reaction (i.e., dissolution) of Al₂O₃ or diffusion of Al³⁺. Increasing Na₂O in SiO₂‐doped samples increases diffusion of Al³⁺ and Al₂O₃ solubility in the liquid, and thus densification increases by 1%. Based on these findings, we conclude that Bayer Al₂O₃ densification can be manipulated by adjusting the Na₂O to SiO₂ ratio.     

Effects of promoters on catalytic activity and carbon deposition of Ni/γ‐Al2O3 catalysts in CO2 reforming of CH4

Catalytic reforming of methane with carbon dioxide was studied in a fixed‐bed reactor using unpromoted and promoted Ni/γ‐Al₂O₃ catalysts. The effects of promoters, such as alkali metal oxide (Na₂O), alkaline‐earth metal oxides (MgO, CaO) and rare‐earth metal oxides (La₂O₃, CeO₂), on the catalytic activity and stability in terms of coking resistance and coke reactivity were systematically examined. CaO‐, La₂O₃‐ and CeO₂‐promoted Ni/γ‐Al₂O₃ catalysts exhibited higher stability whereas MgO‐ and Na₂O‐promoted catalysts demonstrated lower activity and significant deactivation. Metal‐oxide promoters (Na₂O, MgO, La₂O₃, and CeO₂) suppressed the carbon deposition, primarily due to the enhanced basicities of the supports and highly reactive carbon species formed during the reaction. In contrast, CaO increased the carbon deposition; however, it promoted the carbon reactivity. © 2000 Society of Chemical Industry     

EFFECT OF COMPOSITION AND TEMPERATURE ON SOLUBILITY OF IRON OXIDE IN GROUND-COAT ENAMEL GLASS*

The solubility of iron oxide in ground-coat enamel glasses at various temperatures was studied by adding varying amounts of ferric oxide to the milled enamel and giving the mixture a heat-treatment to acquire uniformity without devitrification at the desired temperature. The iron oxide solubility was obtained by finding the breaking point in the curve for iron oxide versus index of refraction. The frit solubilities were obtained at 1400°, 1600°, 1800°, and 2000°F. with variations in Na₂O, B₂O₃, A1₂O₃, CaF₂, CaO, F₂, SiO₂, COO, NiO, MnO₂, BaO, and MoO₃. Data are given on a number of commercial frits.     

X‐ray tomography of feed‐to‐glass transition of simulated borosilicate waste glasses

High‐level waste feed composition affects the overall melting rate by influencing the chemical, thermophysical, and morphological properties of a cold cap layer that floats on the molten glass where most feed‐to‐glass reactions occur. Data from X‐ray computed tomography imaging of melting pellets comprised of a simulated high‐aluminum feed reveal the morphology of bubbles, known as the primary foam, for various feed compositions at temperatures between 600°C and 1040°C. These feeds were formulated to make glasses with viscosities ranging from 0.5 to 9.5 Pa s at 1150°C, which was accomplished by changing the SiO₂/(B₂O₃+Na₂O+Li₂O) ratio in the final glass. Pellet dimensions and profile area, average and maximum bubble areas, bubble diameter, and void fraction were evaluated. The feed viscosity strongly affects the onset of the primary foaming and the foam collapse temperature. Despite the decreasing amount of gas‐evolving components (Li₂CO₃, H₃BO₃, and Na₂CO₃), as the feed viscosity increases, the measured foam expansion rate does not decrease. This suggests that the primary foaming is not only affected by changes in the primary melt viscosity but also by the compositional reaction kinetic effects. The temperature‐dependent foam morphological data will be used to inform cold cap model development for a high‐level radioactive waste glass melter.     

Synthesis of molecular sieves from Chilean kaolinites: 1. Synthesis of NaA type zeolites

The synthesis of NaA type zeolites has been studied at autogenous pressure using Chilean kaolins as starting materials. The ratios of SiO₂/Al₂O₃, Na₂O/SiO₂, H₂O/Na₂O have been set at 1.9 or 2.5, 0.6 or 1.0 and 50 or 90, respectively. Reaction times have been 5 or 15 h and reaction temperature 80 or 100°C. The influence of these parameters on the textural and structural properties of the zeolite has been studied. All samples have been characterized by X‐ray diffraction, infrared spectroscopy, scanning electron microscopy, differential thermal analysis and cation exchange capacity. The best conditions for synthesis are: SiO₂/Al₂O₃=2.5, Na₂O/SiO₂=1.0, H₂O/Na₂O=50 and 15 h reaction time at 100°C. © 1999 Society of Chemical Industry     

White vitreous enamel for ferrous metals with preliminary thermal activation of frit

Vitreous enamels represent a class of inorganic coatings with different properties. In this article, a white vitreous enamel of the Na₂O–B₂O₃–TiO₂–SiO₂ system for enameling of cast iron is introduced. In contrast to the enamel for other metals, the enamel for cast iron must exhibit a high spreading and coefficient of thermal expansion (CTE) of 40–70 mm and 110–120·10⁻⁷ 1/K, respectively. The objective was to obtain an enamel with high whiteness (coefficient of diffuse reflection, CDR not less 80%) and chemical resistance not lower than class A+. This enamel was obtained by correcting the basic composition of the enamel and changing the ratio of SiO₂/B₂O₃ and Na₂O/K₂O oxides. The formulation of the frit contained oxides in the following quantities in wt.%: SiO₂, 43; B₂O₃, 12; P₂O₅, 4; TiO₂, 15; Al₂O₃, 2; MgO, 2; Na₂O, 12; K₂O, 6; and ZnO, 4. This enamel exhibited a set of specified properties. At a temperature of 850 °C, the spreading of enamel was 53 mm, CTE was 110·10⁻⁷ 1/K, and chemical resistance was of class A+. The whiteness property (CDR 83%) was achieved only using an additional technological operation, i.e., the thermal activation of the frit. The thermal activation of the frit at 500 °C for 30 min induced the formation of a fine-crystalline homogeneous coating structure and TiO₂ in the form of anatase. Thermal activation shifts the crystallization temperature to the field of lower temperatures, this is confirmed by the data of the differential thermal analyses. Electron microscopy findings indicated differences in the structure of the enamel after etching in hydrofluoric acid obtained from the frit with and without thermal activation. Preliminary thermal activation of the frit helps obtain a uniform fine-crystalline structure of the coating.     

Phase Equilibria of the Zinc Oxide–Cobalt Oxide System in Air

Phase equilibria of the zinc oxide–cobalt oxide system were studied by a combination of X‐ray diffraction and in situ electrical conductivity and thermopower measurements of bulk ceramic specimens up to 1000°C in air. Rietveld refinement of X‐ray diffraction patterns demonstrated increasing solubility of Co in ZnO with increasing temperature, which is supported by the slight increase in wurtzite (Zn_1?xCo_xO) cell volume and lattice parameter a versus temperature determined for the phase boundary compositions. Similarly, the solubility of Zn in CoO increased with increasing temperature. In contrast, the spinel phase (Zn_zCo_3?zO₄) exhibited retrograde solubility for Zn. Electrical measurements showed that the eutectoid temperature for transformation of rocksalt Co_1?yZn_yO into wurtzite and spinel is 894 ± 3°C, and the upper temperature limit of the stability of the spinel phase is 894°C–898°C for the compositions Co/(Zn+Co) = 0.82–1. The resulting composition‐temperature phase diagram is presented and compared to the earlier (1955) diagram by Robin.     

Sealing Glass‐Ceramics with Near‐Linear Thermal Strain,Part II: Sequence of Crystallization and Phase Stability

The sequence of crystallization in a recrystallizable lithium silicate sealing glass‐ceramic Li₂O–SiO₂–Al₂O₃–K₂O–B₂O₃–P₂O₅–ZnO was analyzed by in situ high‐temperature X‐ray diffraction (HTXRD). Glass‐ceramic specimens have been subjected to a two‐stage heat‐treatment schedule, including rapid cooling from sealing temperature to a first hold temperature 650°C, followed by heating to a second hold temperature of 810°C. Notable growth and saturation of Quartz was observed at 650°C (first hold). Cristobalite crystallized at the second hold temperature of 810°C, growing from the residual glass rather than converting from the Quartz. The coexistence of quartz and cristobalite resulted in a glass‐ceramic having a near‐linear thermal strain, as opposed to the highly nonlinear glass‐ceramic where the cristobalite is the dominant silica crystalline phase. HTXRD was also performed to analyze the inversion and phase stability of the two types of fully crystallized glass‐ceramics. While the inversion in cristobalite resembles the character of a first‐order displacive phase transformation, i.e., step changes in lattice parameters and thermal hysteresis in the transition temperature, the inversion in quartz appears more diffuse and occurs over a much broader temperature range. Localized tensile stresses on quartz and possible solid‐solution effects have been attributed to the transition behavior of quartz crystals embedded in the glass‐ceramics.     

A study of thermodynamic stability of oxide phases in heating multicomponent ceramic binders

A computer simulation of the thermodynamic stability of phases in firing ceramic silicate binders in the SiO₂-A1₂O₃-B₂O₃-K₂O-Na₂O system for abrasive tools based on grained normal electrocorundum is described. The thermodynamic data for the computation are chosen from the HSC 203 data base of the Outokumpu concern and TAPP of the E. S. Microwave Inc. It is established that when the binder (67.73% SiO₂,20.0% A1₂O₃,5.47% B₂O₃, 5.78% K₂O,2.84% Na₂O with CaO, MgO and Fe₂O₃ impurity oxides, the remainder TiO₂) is fired at 1250°C, the most stable phases bear potassium (KAlSi₃O₃ and KAlSi₂O₆), the amount of the sodium-bearing phases diminishes substantially after 900–1100°C with only the Na₂O · A1₂O₃ · 2SiO₂ phase remaining stable. This seems to explain the fact that the proportion of K₂O to Na₂O in feldspar pegmatites and ceramic binders for abrasion tools is regulated by standards.     

Thermal Conductivity of Molten B2O3, B2O3–SiO2, Na2O–B2O3, and Na2O–SiO2 Systems

With the aid of the transient hot‐wire method, the thermal conductivity of molten B₂O₃, B₂O₃–SiO₂, Na₂O–SiO₂, and Na₂O–B₂O₃ systems was measured along with their temperature and composition. It was observed that the thermal conductivity of pure B₂O₃ increased with temperature, until about 1400 K, and then decreased subsequently. Using the MAS‐NMR, 3Q‐MAS, and Raman spectroscopy, the structure of B₂O₃ and SiO₂ in the B₂O₃–SiO₂ system was confirmed. Findings show that an addition of B₂O₃ into the pure SiO₂ system causes a significant decrease in thermal conductivity, due to the formation of boroxol rings. The thermal conductivity of the Na₂O–SiO₂ system was measured and its phonon mean free path was calculated. In addition, a positive linear relation between viscosity and thermal conductivity was observed. In the Na₂O–B₂O₃ system, it was found that a change in the relative fraction of 4‐coordinated boron has an influence on the thermal conductivity when the concentration of Na₂O is between 10 and 30 mol%, in which case the tetraborate unit is dominant.     

Development of a lead‐free copper co‐fired BNT‐based piezoceramic sintered at low temperature

Compatibility of Bi‐based piezoelectric ceramic and copper electrodes is demonstrated by co‐firing 0.88Bi_1/2Na_1/2TiO₃–0.08Bi_1/2K_1/2TiO₃–0.04BaTiO₃ (BNKBT88) with copper. A combination of Bi₂O₃, CuO, ZnO, Li₂CO₃, and B₂O₃ are used as additives to reduce firing temperature to 900°C with minimal effect on the electromechanical properties compared to sintering at 1150°C without additives. Co‐firing with copper electrodes requires controlled oxygen sintering at low temperature. The atmosphere is controlled using carbon dioxide and hydrogen gas to maintain an oxygen partial pressure of 6.1 × 10^?8 atm, which is necessary for the coexistence of Cu metal and Bi₂O₃. The thermodynamic activity of bismuth oxide in BNKBT88 is calculated to be 0.38. BNKBT88 ceramics were successfully co‐fired with internal as well as surface Cu metal electrodes. The copper co‐fired ceramics were successfully polarized and the dielectric and piezoelectric properties are evaluated.     

FRIT SOLUBILITY: II,CONTROLLED VARIABLES OF FRIT COMPOSITION AND THEIR RELATION TO ENAMEL CONSISTENCY *

The sodium carbonate and borax of a sheet-steel ground-coat composition were varied to give variable frit solubility. Definite relationships between enamel consistency and Na₂O and B₂O₃ content of the mill liquor are shown. The ratio of soluble Na₂O to B₂O₃ is of more importance than their total solubility.